Air bubbles in ink flow paths of inkjet printers can impact the performance of the printers. In printers that use solid ink, air bubbles are formed during the freezing and melting of the solidified ink. Typically, when a solid inkjet printer is not operating, melted ink in the ink flow paths solidifies.
FIG. 3A is a cross-sectional view of fluid paths, a pressure chamber, and air vents in a prior art inkjet in a printhead 500, and FIG. 3B is a top plan view of an exemplary nozzle plate 550 in a printhead that includes the inkjet of FIG. 3A. The exemplary print head 500 is configured for use in an inkjet printer. While FIG. 3A and FIG. 3B depict a single inkjet for illustrative purposes, existing printhead embodiments include multiple inkjets, including arrays of hundreds or thousands of inkjets in some embodiments.
In FIG. 3A, the printhead 500 includes a substrate 520, a silicon wafer 530 on an upper surface of the substrate 520, an ink passage 540 through the substrate 520 and silicon wafer 530, a tube 545 connecting the ink passage 540 of the print head 500 to an ink supply reservoir, and a nozzle plate 550 mounted on the structure. An electrostatically actuated membrane 560 is formed on the silicon wafer 530 as shown. A pressure chamber 565 receives liquid ink through the fluid ink passage 540. A nozzle hole 570 and a matrix of purge vents 590 (FIG. 3B) can be formed in the nozzle plate 550. The purge vents 590 in FIG. 3A and FIG. 3B are formed as a group of small nozzle holes formed through the nozzle plate 550. Air enters and leaves the pressure chamber 565 during operation of the print head 500 through the group of purge vents 590. The purge vents 590 are large enough to enable air to escape from the pressure chamber 565 as ink fills the pressure chamber, and to admit air when liquid ink in the pressure chamber solidifies in embodiments of the printhead 500 that use a phase-change ink.
In the print head 500, the membrane 560 is an electrostatically actuated diaphragm, in which the membrane 560 is controlled by an electrode 562. The membrane 560 can be made from a structural material such as, for example, polysilicon, as is typically used in a surface micromachining process. An air vent 564 between membrane 560 and wafer 530 can be formed using typical techniques, such as by surface micromachining. The electrode 562 acts as a counterelectrode and is typically either a metal or a doped semiconductor material, such as polysilicon. Alternative inkjet embodiments include a piezoelectric actuator or a thermal actuator.
During operation of an electrostatic or piezoelectric actuator, the electrode 562 receives an electrical signal and the membrane 564 deflects into the pressure chamber 565. The deformation generates pressure on the ink in the pressure chamber 565 and the pressure urges an ink drop, such as the ink drop 582, through the nozzle 570. In some configurations, the membrane 560 deflects toward the electrode 562 prior to deflection into the pressure chamber 565 to draw ink into the pressure chamber 565 for ejection through the nozzle 570. In a thermal inkjet, the electrical signal generates heat in the pressure chamber and the heat produces an air bubble that urges ink in the pressure chamber 565 through the nozzle 570 to eject an ink drop in a similar manner to the arrangement of FIG. 3A.
The purge vents 590 in the nozzle plate 550 have diameters that are typically smaller than the diameter of the nozzle 570, and are sufficiently narrow to prevent ink from passing through the nozzle plate 550 at a location other than the nozzle 570 during operation of the printhead 500. During operation, a meniscus of liquid ink forms across the opening to each of the purge vents 590 from the nozzle plate 550 to the pressure chamber 565. The strength of the meniscus enables ink to remain in the pressure chamber 565 and to be ejected through the nozzle 570 without being ejected or otherwise leaking through the purge vents 590. In one embodiment, each of the purge vents 590 is formed with a diameter of approximately 3 to 5 microns. In comparison, the diameter of the nozzle 570 is approximately 27.5 microns in the embodiment of FIG. 3A. The small size of the purge vents 590 minimizes the impact of the vents on the flow of liquid ink to the inkjet, which enables the ink to flow to the pressure chamber 565 with sufficient liquid pressure to supply the inkjet with ink during printing operations.
In the prior art embodiment, the vents 590 enable air bubbles to escape from liquid ink in the fluid path 540 and pressure chamber 565. Some air bubble, however, may be formed in portions of the printhead where the air bubbles are unable to be vented easily. For example, in the printhead 500 an air bubble that forms near the nozzle 570 does not escape through the vents 590, but instead escapes through the nozzle 570 where the air bubble disrupts the process of ejecting ink drops. Additionally, while small air bubbles that form near the vents 590 can escape from the printhead 500, larger air bubbles formed within the channel 540 and the pressure chamber 565 can interrupt the flow of ink to the pressure chamber 565 for a longer period of time before escaping from the printhead 500. What is needed is a printhead design that mitigates the formation of air bubbles in locations that are difficult to purge, and mitigates the formation of large air bubbles.